Your gecko's poop is absolutely normal i'm pleased to say! You should find in general, that the black portion is roughly 2/3 times bigger than the white. The white portion of this excretia is your geckos wee wee! Their urine comes out this way in a smaller, white almost chalky deposit. Occasionally, if you use a calcium subsrate ie 'calci sand' then these white deposits can camouflage into the surrounding substrate. Only when you open your viv, and your met with a strong 'public urinal' smell do you know you have a sray 'piss pellet' as I call them.
If your geckos poo is really runny or obviously green in colour, then I would suggest you visit your nearest specialising vetrinary surgeon, taking with you a fecal sample for analysis. These poo's can occure during or following a stressfull period but its always best to visit the vet (with a sample of poo!) to rule out possible viral or parrisitic infection.
Remember.....the best way to avoid many problems people face with their Leo is to adopt clean husbandary practices. Not that I'm suggesting you dont already of course!